Clicking the "Full-screen" button on the compose modal shows the following dialogue; perhaps the author just closed the tip?
Click here to exit full-screen.
(Shift + click to compose in a new window.)
You can set full-screen as your default using
the More menu next to Discard.
Likewise, the tooltip on the "Full-screen" button states Full-screen (Shift for Pop-out). And for someone that spends so much time in email, like a developer and his IDE, it might be time to improve one's efficiency with keyboard coords -- gmail shortcuts (https://support.google.com/mail/answer/6594) -- and use Shift+c (popup) or Shift+d (tab) composition.
